To change the outline – which changes the category order for posts – drag the lines. (On the web, the drag handles will appear to the left of a line when you mouse over it.)
Posts can be similarly be reordered be dragging them around. (Again, on the web, the drag handles appear when you mouse over an entry.)
If you are talking about the pages that are peers to the Outline (created using the "Add New Page…" button), I don't see how they can be reordered. I encourage you to tell Apple about it. (I just did, but duplicates always help.)